1. Field
The present disclosure relates generally to air vehicles and in particular to vehicles that are lighter than air. Still more particularly, the present disclosure relates to a method and apparatus for a lighter than air vehicle capable of carrying cargo.
2. Background
A lighter than air vehicle or aircraft is also referred to as just an airship. Typically, an airship may be steered or propelled through the air using rudders, elevators, and a propulsion system, such as a propeller. Unlike other aerodynamic aircraft, such as airplanes and helicopters, an airship may stay aloft by filling a volume encompassed by a shell with a gas that is lighter than the surrounding air.
Airships may be non-rigid, semi-rigid, or rigid. Non-rigid airships use a pressure level in excess of the surrounding air pressure to maintain a shape. Semi-rigid airships require internal pressure to maintain shapes but usually have frames to distribute suspension loads. Rigid airships may have rigid frames containing multiple non-pressurized gases to provide lift. Rigid airships do not depend on internal pressure to maintain their shape and may be made to virtually any size or shape.
A number of different factors are present in providing lift for an airship. For example, factors include buoyancy, aerodynamic lift, drag, stability and control, and ice and moisture buildup.
Airships may have two forms of drag. The first is often called “parasite drag”. This form of drag results from friction of the air on the surface of the vehicle. The drag is approximately proportional to the surface area of the vehicle times the speed squared. The second form is usually called “induced drag”. This drag arises from the dynamic creation of lift by forcing air downwards, generally by the body in the case of an airship. This drag varies as the square of the lift produced and diminishes as speed increases.
Airships generally derive almost all of their lift from buoyancy and have little aerodynamic lift. As a result, airships have almost no induced drag, and almost all drag is parasite drag. Buoyancy is created by enclosing a volume with less density than that of the surrounding air. This volume is generally filled, at least in part, with a lower density gas. Total buoyancy is equal to the mass of the displaced air minus the mass of the gas within the airship volume. For a given required buoyancy and average gas density, an airship encloses a certain volume.
Because the density of air is relatively low, airships are generally very large compared to airplanes for the same gross weight. Airship gross weight may be defined as the sum of the operating empty weight, payload and fuel. The mass of the enclosed gases is not included. The volume of gas is generally enclosed in an envelope or shell that is shaped to provide a balance between low surface area, such as a sphere, and a very streamlined shape, such as a slender “teardrop”. This balance may result in the relatively chubby shape of a typical airship.
Airships typically have very large surface areas and no induced drag. As a result, airships can fly with almost no drag at very low speeds. Parasite drag, however, increases rapidly with speed. As a result, efficient airships are generally limited to much lower speeds than airplanes. The optimum speed in still air is generally chosen as a balance between fuel efficiency, best at low speed, and productivity, best at high speed. This balance may depend on the cost of fuel and the cost or value of the flight on a money per time basis.
In general, the efficiency of airships is dependent on the buoyancy closely balancing the gross weight. The ability of most airships to create aerodynamic lift is limited by the low flight speed and by the lack of wings. Also, induced drag may increase rapidly with aerodynamic lift.
An airship may provide aerodynamic lift. This lift can be directed to maneuver the airship around a turn or into a climb or descent, for instance. Lift can also be used to balance the difference between airship gross weight and its buoyancy. Airships are infrequently maneuvered, so induced drag losses during maneuvers does not strongly affect overall efficiency. On the other hand, flying with an ongoing mismatch between buoyancy and gross weight may provide a constant induced drag force that can detract from the vehicle's efficiency. As a result, the need for airship buoyancy to closely match airship gross weight can be diminished if the airship can create aerodynamic lift with an acceptable measure of efficiency.
It is also a general goal to reduce the drag force of airships so that less fuel is burned. Alternatively, it is desirable to reduce drag so that the airship may fly faster with the same fuel consumption.
The size of an airship influences relative drag. This drag may result from the square cube law. The gross weight of an airship is proportional to the displacement (volume) of the body (envelope). This volume is proportional in turn to the cube of the body's length. Drag force is approximately proportional to the surface area of the body which is in turn proportional to the length of the body squared. Thus, the drag per unit gross weight at a given speed drops approximately according to 1/length.
Airship weight and drag may be tightly related. Weight determines the body volume needed to provide buoyancy. The size of the body, in turn, contributes significantly to the weight of the airship.
In general, weight may also be reduced by increasing the efficiency of all airship systems. For instance, a more efficient propulsion system may consume less fuel. This efficiency reduces gross weight. As a result, the body size of the airship may be reduced. In turn, drag and fuel burn may be reduced until the size converges.
With respect to stability and control, a number of parameters may affect the stability and control of airships. For example, when an airship is traveling a straight and level flight at a constant altitude, several conditions are needed. This type of flight is also referred to as a cruise mode. With this type of mode, the lift equals the weight where the lift is the sum of the buoyancy and aerodynamic lift and the weight is the gross weight.
The thrust equals the drag and all moments are required to be zero. The sum of the moments about each axis must equal to zero to provide this type of flight. Further, with a laterally symmetric airship, the roll and yaw moments are typically zero.
A laterally offset payload or fuel load may result in some rolling moments. In a similar fashion, a longitudinally offset payload results in a pitching moment. A lateral offset may result in listing of the airship to one side. These are just some non-limiting examples of the type of parameters that are needed to provide stability.
Further, it is also desirable to be able to change the flight direction orientation of the airship during flight. Further, it is desirable for some airships to be able to maneuver at low speeds or at zero speed. To fly at zero speed, the airship may be neutrally buoyant. Alternatively, the airship may be close to neutrally buoyant with the difference between buoyancy and weight being made up by a vertically-oriented propulsion system. At zero speed, aerodynamic lift on the airship shell is zero, so it cannot make a contribution to lift.
Another factor affecting airships is the build up of ice and moisture. Some airships may build up ice depending on weather conditions. Additionally, certain materials used in an airship may absorb water. These types of conditions may influence the weight of the airship.
Therefore, it would be advantageous to have a method and apparatus that overcomes one or more of the problems described above.